Interesting problem. More Z-Wave Plus components are in the pipeline, but I am not aware of many intended for outdoor use. Remember that most powered Z-wave devices (outlets and switches) will also function as Z-wave repeaters, so don’t limit your search to z-wave repeaters.
If you currently open the gate using a remote control, you may be able to just add an intermittent z-wave switch. This can be done programmatically by changing a standard z-wave switch into an intermittent switch (turns on and then off automatically). This was what was done within SmartThings before z-wave garage door openers were commercially available. I know that Aeon Labs has a Nano Dimmer in the pipeline that is designed to go inside of electrical boxes behind existing switches. Housing this device inside a plastic waterproof box should provide sufficient protection in an outdoor environment. A z-wave switch to open and close the gate would also function as a repeater for a battery powered Z-Wave open/close sensor. Since most open/close sensors are intended for indoor use, they would probably require additional weather proof housing as well.
Another alternative would be to run a line from the house to the gate.